Biomass, such as forestry and agricultural products and residues, is a major underutilized product in the world. There are several different technologies for converting the biomass to useful energy (e.g., direct burn, co-firing, gasification, and the like) or to biobased products (e.g., fermentation, pyrolysis, and the like), in particular bio-oil. Depending on the type of process used, the final product may have different values and applications. In most cases, these products replace those generated from crude oil, thus having long-term substainability and environmental benefits (e.g., being carbon neutral).
Fast-pyrolysis oil, also known as bio-oil, is one type of renewable liquid fuel produced from agricultural or forestry residues. Generally speaking, bio-oil has several advantages in terms of energy independence, environmental friendliness, and produce cost. Nonetheless, two main drawbacks of bio-oil limit its usage as high-grade/transportation fuel (Huber et al., “Synthesis of Transportation Fuels from Biomass: Chemistry, Catalysts, and Engineering,” Chem. Rev. 106(9):4044-4098 (2006); Czernik et al., “Overview of Applications of Biomass Fast Pyrolysis Oil,” Energy Fuels 18(2):590-598 (2004); Zhang et al., “Review of Biomass Pyrolysis Oil Properties and Upgrading Research,” Energy Conyers. Manage. 48(1):87-92 (2007); and Qasmaa et al., “Fuel Oil Quality of Biomass Pyrolysis Oils—State of the Art for the End User,” Energy Fuels 13(4):914-921 (1999)). These drawbacks include (1) high acidity (pH of 2-3), which causes the corrosion of instruments, and (2) high oxygen content (35-40 wt. %), which decreases the heating value of bio-oil to only 16-19 MJ/kg, a value much lower than that of traditional petroleum fuels.
The high acidity of bio-oil can be attributed to the large quantities of carboxylic acids found in bio-oil, such as acetic acid, formic acid, and butyric acid (Qasmaa et al., “Fuel Oil Quality of Biomass Pyrolysis Oils—State of the Art for the End User,” Energy Fuels 13(4):914-921 (1999); Branca et al., “CG/MS Characterization of Liquids Generated from Low-Temperature Pyrolysis of Wood,” Ind. Eng. Chem. Res. 42(14):3190-3202 (2003); Qasmaa et al., “Fast Pyrolysis of Forestry Residue. 2. Physicochemical Composition of Product Liquid,” Energy Fuels 17(2):433-443 (2003)). For instance, the bio-oil made from cornstalks has a high content of acetic acid (more than 27% based on gas chromatographic-mass spectrometric (GC-MS) analysis) (Zhu et al., “Analyses and Properties of Pyrolytic Bio-Oil from Cornstralk,”J. Univ. Sci. Technol. China 36(4):374-377 (2006)). When AL-MCM-41 is added to biomass as pyrolysis catalyst, the yield of acetic acid can be raised 2-fold (Adam et al., “Pyrolysis of Biomass in the Presence of AL-MCM-41 Type Catalysts,” Fuel 84(12-13):1494-1502 (2005)). On the other hand, bio-oil subjected to fractionation with water (Scholze et al., “Characterization of the Water-Insoluble Fraction from Pyrolysis Oil (Pyrolytic Lignin). Part I. PY-GC/MS, FTIR, and Functional Groups,” J. Anal. Appl. Pyrol. 60(1):41-54 (2001); Sharma et al., “Catalytic Upgrading of Pyrolysis Oil,” Energy Fuels 7(2):306-314 (1993)) or ethyl acetate (U.S. Pat. No. 4,942,269 to Chum et al.) will generate two phases, one of which is also composed of a large amount of carboxylic acids. Furthermore, the carboxylic group is the most oxygen-abundant functional group in bio-oil. The existence of carboxylic acids in bio-oil will lead to the consumption of a large amount of hydrogen during hydrotreatment. A higher temperature is also required for the hydrotreatment, because carboxylic acid is more difficult to hydrogenate as compared with aldehydes, ketones, and alcohols (Elliott, D. C., “Historical Developments in Hydroprocessing Bio-Oils,” Energy Fuels 21(3):1792-1815 (2007)).
Bio-oil is a mixture of water, light volatiles, and non-volatiles and is highly reactive because of the presence of significant quantities of oxygen. Therefore, the common method of distillation (as performed with crude oil) for separation of fractions is not effective. During distillation, the oils start boiling below 100° C., accompanied by numerous polymerization reactions, and distillation stops around 250 to 280° C., leaving as much as 50% of the starting material as residue.
The ability to cool the bio-oil from process temperatures around 450° C. or higher and simultaneously fractionate it would yield a variety of useful products. It is known from previous experience that slow condensation (e.g., condensation that takes place over a time period greater than 2 seconds) can result in reactions between compounds, thus increasing the tar fraction of the condensed bio-oil. In addition, using condensation coils provides surfaces on which tar could deposit and further enhance (catalyze) tar formation.
Commercial bio-oil condensers currently in use are designed to rapidly cool the bio-oil vapors produced during bio-mass pyrolysis to prevent secondary reactions from occurring that convert the vapors into undesired lower molecular weight compounds, char, and gaseous products. Most of the existing bio-oil systems employed commercially are based on single stage spray quenching. The single stage condensers use a single vessel for direct contact heat exchange between gases, vapors and aerosols entering from the pyrolyzer and a spray of cold liquid bio-oil or hydrocarbon. The pyrolysis products are quickly cooled by the cold spray causing vapors to condense. Aerosols produced by this process are washed out of the gas stream by the spray droplets and collected as liquid at the bottom of the tank. Some lab-scale systems attempt to employ multistage condensers. These systems use several vessels in series to cool the bio-oil vapors in stages where the condensed bio-oils are either collected in a single common tank or in individual tanks Both of these condensing systems are typically optimized to condense the bio-oil vapors as quickly as possible using a low temperature coolant (<20° C.) where little thought is given to how the phases are collected. Previous attempts to optimize these lab-scale multistage condensing systems to fractionate the bio-oil have fared poorly due to the use of this low temperature coolant which causes the high boiling point compounds to congeal and solidify on the walls of the condenser tubes where they are subsequently converted into char which over time leads to char buildup eventual blockage of the condenser tubes. Therefore, there is a need in the industry for an alternate method of condensing the bio-oil.
The present invention is directed to overcoming these and other deficiencies in the art.